1. Field of the Invention
The present invention relates to a semiconductor substrate provided with a positioning means such as a so-called orientation flat (OF) or notch and to a semiconductor device made by using such a substrate.
As time passes, semiconductor devices tend to be more densely integrated, operated at a higher speed and made smaller in size at an increasing speed and accordingly the size of a pattern of elements constituting the semiconductor device is reduced, which requires a wafer to have a flat surface and a uniform thickness of 1 micrometer or less. Further, an approach using a substrate composed of two silicon wafers bonded through an insulating layer is most promising at present for providing a semiconductor device of high performance based on a so-called silicon on insulator structure (SOI) technology, in which one of the silicon wafers of the substrate must be thinned to a few micrometers.
2. Description of the Related Arts
A semiconductor wafer arranged as described above is usually made by the process shown in FIG. 1, wherein (a) an ingot of a single crystal semiconductor having a desired composition is pulled up by a Czochralski method; (b) the ingot is roughly cut to a suitable length; (c) the outer surface of the ingot is processed to have a cylindrical shape and further a plane parallel to the axis of the cylinder is formed on the side thereof, the plane serving as an orientation flat (OF) into be described later; (d) the above ingot is sliced to many disks by a rotary blade type slicer; (e) the circumference thereof is beveled; (f) at least the surface thereof where a semiconductor device is to be formed is subjected to lapping; (g) the disk is etched with a chemical solution; (h) the disk is then mirror finished by a so-called chemical-mechanical polishing; and finally (i) the disk is cleaned. The mirror finished surface has flatness and crystallinity suitable to form a semiconductor device. The semiconductor wafer made by the above process is a disk having a straight cutout or orientation flat (OF) defined along a portion of the circumference thereof.
Although the etching at step (g) is carried out to remove crystal defects made on the wafer surface by the lapping at step (f), the flatness of the disk becomes deteriorated or sloped by a different etching speed on the surface thereof caused by the nonuniform circulation or distribution and varying temperature of the chemical or etching solution. However, the flatness is recovered by the polishing at step (h).
Nevertheless, the flatness of the semiconductor wafer made by the above process is limited to about 2 micrometers in terms of the value of a total thickness variation (TTV) which is a difference between the maximum and minimum thickness of the wafer. Accordingly, the yield of wafers which satisfy the requirement of the flatness of 1 micrometer or less as described above is very low. This makes it difficult for bonded wafers to be uniformly thinned to a few micrometers or less in the process of making an SOI structure.